1. Field of the Invention
The present invention relates to a polysilicon thin film transistor liquid crystal display (polysilicon TFT LCD), and more particularly, to a polysilicon thin film transistor liquid crystal display having a plurality of common voltage drivers.
2. Description of the Prior Art
Liquid crystal displays (LCDs) have been widely applied to a variety of information products, such as notebook computers and PDAs, because of their small size, low power consumption, and low radiation emission. Liquid crystal molecules are characterized by being capable of allowing different amounts of light to pass according to their rotation angles. Consequently, a liquid crystal display is able to generate rich and colorful images.
Currently, most LCDs are fabricated by an amorphous silicon thin film transistor process, and drivers for controlling the LCD are produced in the form of external ICs connected to the LCD panel. Nevertheless, as technological improvements are made, more and more LCDs are fabricated by polysilicon thin film transistor processes, which result in better display effects (e.g. higher resolution). In addition, related drivers and interface circuits are also integrated into the panel in the form of polysilicon thin film transistors so that the production cost is substantially reduced.
Generally speaking, the above-mentioned integrated polysilicon TFT LCD comprises a common voltage driver. The common voltage driver is normally a voltage follower having an operational amplifier. The conventional polysilicon TFT LCD has only one single voltage follower functioning as a common voltage driver. However, it is easy for the panel to have a non-uniform common voltage distribution because of the resistance-capacitance (RC) impedance of the common electrode conductive wires. This non-uniform distribution of the common voltage leads to poor display qualities. Refer to FIG. 1, which is a schematic diagram of a conventional polysilicon thin film transistor liquid crystal display 10. The polysilicon thin film transistor liquid crystal display 10 comprises a panel 12 having related drivers and interface circuits formed therein. The polysilicon thin film transistor liquid crystal display 10 further comprises a pixel region 14, a first data line driver 16A, a second data line driver 16B, a scan line driver 18, a common voltage driver 20, a timing control circuit 22, and an interface circuit 24. The pixel region 14, the first data line driver 16A, the second data line driver 16B, the scan line driver 18, the common voltage driver 20, the timing control circuit 22, and the interface circuit 24 are formed in the panel 12 in the form of polysilicon thin film transistors.
As shown in FIG. 1, an image signal is first transmitted to the polysilicon thin film transistor liquid crystal display 10, and transferred to related logic circuits such that corresponding images are displayed in the pixel region 14. The pixel region 14 comprises a plurality of display cells, each display cell being a pixel or a sub-pixel and driven by the first data line driver 16A, the second data line driver 16B, and the scan line driver 18. In addition, the timing control circuit 22 generates a timing signal for operating the first data line driver 16A, the second data line driver 16B, the scan line driver 18, and the interface circuit 24. The common voltage driver 20 is for providing a common voltage. However, due to the resistance-capacitance impedance of the conductive wires, non-uniform common voltage distribution readily occurs in the conventional polysilicon thin film transistor liquid crystal display 10.
Refer to FIG. 2, which is a timing diagram of common voltage Vcom of the polysilicon thin film transistor liquid crystal display panel 10 shown in FIG. 1. FIG. 2 illustrates the waveform of the common voltage Vcom of the common voltage driver 20 at the common voltage driver 20, point “A”, and point “B” of the pixel region 14 shown in FIG. 1 respectively from top to bottom. As shown in FIG. 2, the common voltage Vcom is an alternating voltage, and therefore applied to the common electrode of each display cell by means of swing. However, since the lengths from the common voltage driver 20 to point “A” and to point “B” are not equal, and the resistance-capacitance effect is not negligible, the common voltage Vcom at point “A” and point “B” are delayed and decayed compared to the common voltage Vcom in the common voltage driver 20. When the delay or decay makes the phase and amplitude difference of Vcom at point “A” and point “B” too large, display quality of the polysilicon thin film transistor liquid crystal display is deteriorated.